Internal combustion engines combust an air and fuel mixture within cylinders to generate engine torque. During part-load conditions, a percentage of the engine torque may be held in reserve (known as torque reserve) so as to improve an immediate response of the engine during a transient increase in torque demand. In the absence of torque reserve, engine response time may be slower and drivability may be adversely affected.
Engine control systems may provide a demanded torque reserve by adjusting one or more torque affecting engine parameters such as engine air flow and spark timing. Partially closing an intake air throttle reduces the amount of air available to the engine for combustion, generating an air reserve. Likewise, retarding spark timing results in a later combustion which generates a spark reserve. The air and spark reserves are used for generating the torque reserve. As engine operating conditions change, the torque reserve value may be varied. Other parameters that may be actuated to affect the torque reserve include valve timing and fuel injection timing.
One example approach for adjusting an engine torque output including an amount of torque held back in reserve is shown by Pochner et al. in US 2015/0275771. Therein, each of an intake and exhaust valve phaser, an exhaust waste gate valve coupled to an intake turbocharger, and an intake throttle valve are adjusted based on an expected future increase in torque request. The parameters are adjusted to create a fast torque reserve that is based on the anticipated change in torque.
However, the inventors herein have recognized potential issues with such systems. As one example, in the approach of Pochner, the torque actuator adjustments are based on an anticipated change in torque. A larger torque reserve may be created in anticipation of a larger or faster increase in requested torque. However, if the actual increase in requested torque is smaller or slower, torque is wasted. Specifically, spark reserve may be over scheduled and, since spark reserve is immediate, there may be a loss in efficiency if all the reserve is not used. As another example, use of any torque reserve sacrifices engine efficiency during part load operation for drivability. In particular, the response and “feel” of the engine is improved at the cost of the overall efficiency.
In one example, the issues described above may be addressed by a method for an engine comprising: reducing a speed of a turbocharger via an electric motor while increasing an intake throttle opening and advancing a spark timing, a degree of the increasing the intake throttle opening and advancing the spark timing based on the reduced speed of the turbocharger and a desired torque reserve. In this way, at least a portion of torque reserve can be provided via an intentionally slowed down turbocharger in lieu of air and spark reserve.
As one example, an engine may include an electrically assisted turbocharger having an electric motor coupled to the compressor and/or the turbine of a turbocharger. By operating the electric motor, torque reserve can be additionally provided via adjustments to turbine speed. For example, during part-load boosted engine operation, the turbine speed can be intentionally decreased (below the turbine speed that would naturally occur at the current settings) by operating the motor in a regeneration mode. The reduction in turbine speed reduces the manifold pressure and thereby the air mass ingested into the engine. This allows for the engine to be operated with a smaller air reserve, by holding the throttle more open, and smaller spark reserve, by applying a smaller amount of spark retard. For example, instead of scheduling a 5% air reserve by closing the throttle by an additional amount to provide a corresponding throttle plate delta pressure (TPDP), regeneration through the negative motor torque is used to slow the turbocharger to a speed corresponding to a 5% reduction in air mass. At the same time, the throttle may kept wide open (WOT). As a result, better control of transient manifold filling is provided without the efficiency penalty associated with TPDP because energy can be regenerated in the process.
In this way, adjustments to an electric assisted turbocharger can be coordinated with throttle and spark adjustments to provide a desired torque reserve. By reducing turbine speed at part-load engine operation via an electric motor operating in a regeneration mode, an air mass provided to the engine can be reduced. The improved manifold filling improves part load drivability as the manifold fills faster and torque response is smoother. By providing at least a portion of the torque reserve via the electric turbocharger, the reliance on spark and throttle reserve is reduced. In addition, efficiency losses associated with over scheduling spark reserve are reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.